Microprocessor-based controls are used with automotive vehicle engines to perform various functions. The invention relates to an algorithm for the microprocessor that enables a vehicle to maintain, or hold, speed on a grade. While this might at first blush appear to be a speed control or cruise control function, this grade-holder speed control aspect of the invention is conceptually different from the usual speed control or cruise control because it operates without the driver being required to invoke a speed control or cruise control mode of operation for the vehicle.
A diesel engine which comprises electronic speed governing has the ability to operate on any particular one of a number of different horsepower vs. speed curves. The grade-holder speed control of the present invention comprises an algorithm in the microprocessor which is effective to switch the engine operation from a lower horsepower vs. speed curve to a higher one when an increase in road grade which initiates an incipient reduction in vehicle road speed is detected. The algorithm causes the engine operation to switch to a higher horsepower vs. speed curve so that vehicle speed is maintained despite the increase in grade. In this way it is possible to improve the fuel economy by allowing the vehicle to operate on a more economical, lower horsepower vs. speed curve for level road conditions (i.e., zero grade), yet to operate along higher horsepower vs. speed curves on increasing grades. Driveability is also improved because fewer downshifts will be required.
One advantage of the grade-holder speed control of the present invention is that it can be implemented on certain known vehicle systems without hardware modifications, the only modification being a software change. It can also respond to other factors that change the load on the vehicle, such as a headwind for instance.